Nope, this is no existential question (although...). Everybody's talking about "When will Bitcoin die", but what if you die, and your family don't find your key, or don't understand it, or think it's only a useless sheet of paper with some code on it, or simply throw it away... Does this mean the currency's gone forever?
I find the possibility of this event rather realistic (yup I don't believe in eternal life), so that means that someday there might be 21M BTC but only 18M (and counting down...) is left in the market?
Maybe you are right. If someone die and their wallet remain untouch, then the balance will sit there forever.
total supply may not change, but circulation will reduce eventually. I think the best way to prevent this is to store your wallet information,
and save it like a will for your kid or families or relatives.